根据当前的日期获取到周起始，月起始，年起始

一、根据当前的日期获取到周起始和截止

 1 /// <summary>
 2 /// 得到本周第一天(以星期天为第一天)
 3 /// </summary>
 4 /// <param name="datetime"></param>
 5 /// <returns></returns>
 6 public DateTime GetWeekFirstDaySun(DateTime datetime)
 7 {
 8     //星期天为第一天
 9     int weeknow = Convert.ToInt32(datetime.DayOfWeek);
10     int daydiff = (-1) * weeknow;
11 
12     //本周第一天
13     string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
14     return Convert.ToDateTime(FirstDay);
15 }

得到本周第一天(以星期天为第一天)